It is necessary to carry out a synchronisation process in the majority of telecommunication systems so as to adequately obtain the information transmitted from the received signals to the channel. One or more types of synchronization are required depending on how the transmission is carried out and the modulation used. In general, to transmit using OFDM modulation it is necessary to carry out synchronisation in time which consists of determining the start of the OFDM symbols in reception, and synchronization in frequency, so that frequency of the oscillators used for the sample or for the analog traslation of transmitted and received signal is sufficiently close.
Many methods of synchronisation of OFDM signals are known in the state of the art but none of these are secure enough to avoid false indications of synchronisation in point to multipoint systems in which the electricity network is the transmission means used.
It is well known that use of the electricity network as a transmission means is problematic for synchronisation since the connection-disconnection of various apparatus on the network produces tension peaks and impedance variation on the line causing channel response to vary in time. Impulsive noises, which are very usual in the electricity network affect synchronisation, in large part because by definition these are occasional noises that affect a small number of samples and therefore can impede adequate detection of the synchronisation sequences if they coincide in time with said sequences. Band separation as proposed in this patent, reduces the consequences of these noises so that it is possible to apply known synchronization methods, which due to the improvement proposed will offer optimum results in noisy environments such as the electricity network.
Among the synchronisation methods in the state of the art those that particularly stand out are described in U.S. Pat. No 5,732,113 which describes a process of time synchronisation that uses a single synchronisation symbol with two equal half symbols, and that described in Spanish patent submission 200.101.121 which refers to a “process for the synchronisation in the down stream of various uses in a point to multipoint transmission system with OFDM modulation”, and which describes the transmission of two identical synchronisation symbols. The present invention improves synchronisation in the time and frequency domains due to synchronisation in various bands and achieves results with less variance and better adjusted to the real value than that which can be achieved by applying either of the previously mentioned process. Furthermore, the application of the method proposed allows synchronisation to be carried out in many cases where noise on the line and channel characteristics would prevent the achievement of any result when using known synchronisation methods.
On the other hand, it is important to indicate that the filter banks such as those described in the book “Multirate Systems and Filters Banks”, by P. P. Vaidyanathan, published by Prentice Hall in 1993, are known but that these filters have not been used for the division of synchronisation sequences received for independent detection in frequency bands such as described in this invention and which achieves the considerable improvement over the state of the art as described above.